DIY Electric Car Forums banner

1 - 8 of 8 Posts

·
Registered
Joined
·
4 Posts
Discussion Starter #1
Good afternoon all!
Been lurking here for a few months, registered just recently so I could start asking questions. I was directed to read up here by Merritt from EVWest after I went to visit to get an idea of what they do.

Since I'm still a ways from being able to start a project car, I figured I'd use the time to do the math and design work before dumping in the money and time. I am aiming for a daily-driver type vehicle with about 200 miles of range, and capable of highway speeds (80 mph). While cost is worth keeping in mind, I am of the opinion that cost is more of a delay than a limiter. So, here's what I have put together so far:

The donor car's weight (picking a toyota 86 for a start, since I like the design and there's always a supply of them down here) is 1298 kg base, from which I anticipate removing ~246kg (Engine, transmission, spare tire, gas tank...).
Next, just for ease of access, I put together the tesla model s drive unit (everyone loves) with the model s batteries. Not sure if other important components affect the performance which aren't in the package, but we'll get there.

Given the motor is rated for up to 400 V, 400 kW, and 1k A, I arranged the batteries in a 2p10s configuration for (2*500) 1k Ah and (10*22.8) 228 V, with a total kWh of (20*5.3) 106. Though I was unable to find on this forum the difference between a high V low A setup or a low V high A setup. (Basically, wondering how a 1p20s layout would affect things).

Using the required power formula from the introduction section:
(mass * g * V * RollRes) + (drag * area * V^3 * 0.6465)
I set V to be 80 mph, used a roll resistance (0.015) and drag coefficient (0.27) from wikipedia, and set the cross-sectional area to just be a rectangle of the maximum dimensions (2.28 m^2). The mass I then estimated to be (1298-246 +642) kg (accounting for motor kit and batteries).
This resulted in a required power of 27.117 kW, and a Wh/mi of 338.9.
Further, dividing this Wh/mi by my total kWh gives me roughly 312 miles of range.


So, with all that covered:
1) The wiki was not exactly clear on how to calculate top speed, would this require finding a spec graph for the motor and using my parameters?

2) Did I over-design this a bit? The motor seems to require a minimum 200 volts and I worry that cutting it to a 2p8s setup might not be sufficient for it to run.


3) If the voltage does matter for top speed, are there any ways to increase it (short of more batteries, or going to the single-series config)?


4) Lastly, I've noticed most of the donor vehicles here tend to be older models. Is that just a community preference, or are the safety features that big of an issue for these types of projects?


Glad to have found this place!

 

·
Registered
Joined
·
113 Posts
Your country, your vote. Depending on location you may have to have all the safety features the car originally care with. On newer models with advanced safety mechanisms that are ‘can’ controlled with the whole car may become problematic
Your battery spec is and always will be confusing as generally speaking the configuration is at cell level regardless of how they are contained in the modules. So 10s in Tesla money is 36v being each cell is 3.6v. I think a lot of thought will be required in battery placement, a lot of modules at the front and a heavy motor on the rear is probably not a good idea. I am surprised more high waisted cars are not being used to allow a floor plan design. Saab 900 turbo comes to mind. To old and size becomes a issue (footprint) as early cars has small interiors as space was never a premium, otherwise you would simply re-shell a model S and be done. Modern cars and as people are finding out Tesla’s a wide cars. Most 80’s cars are very narrow by comparison with setups for small tires and 14 inch wheels if you are planning for some additional traction you will be hunting for room quickly.
For long distances I guess you will be utilizing the Tesla model S AC (a few early ones have Can messages already decoded) and coolant system along with the BMS design, there is a bms board that will control the module bmb’s in the modules. Thermoregulation is super important in Tesla modules.
 

·
Registered
Joined
·
113 Posts
People are moving from higher amps to higher volts as you produce a lot less heat at a given kW, 400v systems are the new norm but in a few more years that may well be 800v but for now I would feel safe in committing to 400v for HVAC, dc/dc converters, charging inverters and connectors.
 

·
Administrator
Joined
·
6,120 Posts
Hi
Getting 106 kWh of batteries into a small car is going to be difficult!

I have 14 kWh in my car - I could get another 7 kWh in it at a push
 

·
Registered
Joined
·
4 Posts
Discussion Starter #5
x.l.r.8: Good to know, I read a few instances of heat issues here, I'll start evaluating having a straight 16 then (1p16s). I'll also start looking into those control boards as well.


Duncan: Yea, and it amazes me how people have managed to cram them in other builds (like that Audi a few months back). I need to get a 3d model (regardless of accuracy) and test out some arrangement ideas.
 

·
Registered
Joined
·
197 Posts
Using the required power formula from the introduction section:
(mass * g * V * RollRes) + (drag * area * V^3 * 0.6465)
I set V to be 80 mph, used a roll resistance (0.015) and drag coefficient (0.27) from wikipedia, and set the cross-sectional area to just be a rectangle of the maximum dimensions (2.28 m^2). The mass I then estimated to be (1298-246 +642) kg (accounting for motor kit and batteries).
This resulted in a required power of 27.117 kW, and a Wh/mi of 338.9.
Further, dividing this Wh/mi by my total kWh gives me roughly 312 miles of range.


So, with all that covered:
1) The wiki was not exactly clear on how to calculate top speed, would this require finding a spec graph for the motor and using my parameters?

2) Did I over-design this a bit? The motor seems to require a minimum 200 volts and I worry that cutting it to a 2p8s setup might not be sufficient for it to run.

3) If the voltage does matter for top speed, are there any ways to increase it (short of more batteries, or going to the single-series config)?

4) Lastly, I've noticed most of the donor vehicles here tend to be older models. Is that just a community preference, or are the safety features that big of an issue for these types of projects?
1) One type of top speed is when the power required to sustain speed is equal to the maximum power the drivetrain can deliver. If you only had 27kW, you could theoretically cruise at 80mph, but would take a very long time to reach vMax. The next type of top speed is when the motor or gearing isn't rated to spin any faster. The last type is less definable, but is the point when the road conditions and aerodynamic behaviour become too much for the tyres, suspension or driver to handle.

2) The motor may turn with less volts, but with Tesla drive units you tend to keep the power electronics within, so they may have a low-voltage cutoff.

3) Current correlates with torque. As motor speed rises, you need more voltage to deliver the same current due to back-EMF. Running a lower battery voltage means your torque curve will drop at lower revs, so your peak power and top speed will be lower. The last 3 sentences are a gross simplification of what's actually happening inside motors. Theoretically, you could have a 100v high-current motor and a 600v low-current motor with exactly the same power capability, but if you already have the motor you can't change the nominal voltage and keep power the same unless you modify the internals of the motor by changing at least the windings.

4) Older cars are cheaper and simpler. The latter is important as the documentation for modern auto electronic systems is rarely freely available to the public. It's easy to generate the pulses and voltages needed to make an analogue dashboard work, CAN-based modern systems can be easy once you've worked out which message codes and values trigger each function of the dash. Early digital systems can be a problem since you don't necessarily know what protocol the signals are even using. Some jurisdictions prohibit removal of safety systems such as ABS, airbags, traction/stability control, so it's easier to just start with a car which never had those systems.
 

·
Registered
Joined
·
4 Posts
Discussion Starter #7
1) One type of top speed is when the power required to sustain speed is equal to the maximum power the drivetrain can deliver. If you only had 27kW, you could theoretically cruise at 80mph, but would take a very long time to reach vMax. The next type of top speed is when the motor or gearing isn't rated to spin any faster. The last type is less definable, but is the point when the road conditions and aerodynamic behaviour become too much for the tyres, suspension or driver to handle.
This is good to know; so with a 27 kW requirement, and an available ~228 kW, then I should expect for this configuration relatively easily, it sounds like.

2) The motor may turn with less volts, but with Tesla drive units you tend to keep the power electronics within, so they may have a low-voltage cutoff.

3) Current correlates with torque. As motor speed rises, you need more voltage to deliver the same current due to back-EMF. Running a lower battery voltage means your torque curve will drop at lower revs, so your peak power and top speed will be lower. The last 3 sentences are a gross simplification of what's actually happening inside motors. Theoretically, you could have a 100v high-current motor and a 600v low-current motor with exactly the same power capability, but if you already have the motor you can't change the nominal voltage and keep power the same unless you modify the internals of the motor by changing at least the windings.
Noted, so this plays back to what x.l.r.8 was referencing, and reinforces the idea of a straight series battery config for a higher voltage.

4) Older cars are cheaper and simpler. The latter is important as the documentation for modern auto electronic systems is rarely freely available to the public. It's easy to generate the pulses and voltages needed to make an analogue dashboard work, CAN-based modern systems can be easy once you've worked out which message codes and values trigger each function of the dash. Early digital systems can be a problem since you don't necessarily know what protocol the signals are even using. Some jurisdictions prohibit removal of safety systems such as ABS, airbags, traction/stability control, so it's easier to just start with a car which never had those systems.
This is a good point. My main reason for picking a more recent donor car was to avoid the issues of bodywork and repair (from age, rust, etc).

There is a lot of good info here, and I'm sensing there's more tribal knowledge that probably hasn't been codified yet into the wikis. That, and I would really benefit from working with someone who has already done one of these before. I really appreciate the answers!
 

·
Administrator
Joined
·
6,120 Posts
Picking a donor car

There is no point in picking a "normal car" to convert - the conversion will cost a LOT more than a second hand production EV and will be nowhere near as good

Think of this as "Hot Rodding" - you need to end up with something SPECIAL to make it worth while
Which is another reason why people choose older "classic" cars
 
1 - 8 of 8 Posts
Top